1. Field of the Invention
A standard technique for assessing solid or liquid aerosol concentrations is by abstracting a quantity of air and measuring the aerosol fraction. The aerosol can be measured by a variety of methods. One method is to collect the aerosol fraction of the air sample on a filter, with either subsequent determination of collected mass, examination by microscopy, or analysis by chemical methods. Other methods for aerosol measurement involve the use of sensors, but in most cases the principle of sample abstraction is the same. The flow rate through a sampler is usually established according to the specifications of the sampling method and is held constant so that an accurate sample and representative volume can be determined. Stationary aerosol samplers are used to evaluate both outdoor and indoor environments including work-places. Personal breathing zone samplers worn by workers are used to estimate their exposures to workplace pollutants. The inlets of commercially available samplers have evolved to measure the aerosol concentrations in different environments. Significant biases (primarily particle losses) may occur during aspiration into a sampler and during transmission of aerosols through the sampler. These biases are sensitive to the magnitude and direction of the ambient air velocity. In indoor work environments, where the air velocity typically ranges from 100 to 300 cm s.sup.-1, the external geometry of a sampling cassette may influence the flow pattern in the vicinity of the sampler's inlet, thereby adversely affecting the sampler's performance.
The sampling efficiency of a sampler, E.sub.s is defined as the ratio of the sampled particle concentration, C.sub.s, to the environmental particle concentration, C.sub.o EQU E.sub.s =C.sub.s /C.sub.o ( 1)
To determine C.sub.o, it is important that the inlet efficiency be evaluated under a range of controlled operating conditions. Particle size distribution, wind velocity U.sub.w, inlet velocity U.sub.i, inlet shape, particle density, and inlet orientation with respect to the wind and gravitational force are some of the factors which affect sampling efficiency.
For isoaxial sampling, the velocity ratio R, which is the ratio of the wind to the inlet velocity, determines whether the sampling is isokinetic (R=1), subisokinetic (R&gt;1) or super-isokinetic (R&lt;1). During isokinetic sampling, the limiting stream-surface flows into the inlet without a change in direction, and the particle concentration at the face of the inlet is equal to C.sub.o. During non-isokinetic aspiration, particle inertia may lead to the migration of some particles across the limiting stream-surface, resulting in a different aerosol concentration at the face of the inlet.
For the simple case of a tubular, thin-walled inlet, overall sampling efficiency consists of two major components--aspiration efficiency, E.sub.a, and transmission efficiency, E.sub.t : EQU E.sub.s =E.sub.a E.sub.t ( 2)
Because of the complex geometry of many aerosol samplers and the unstable wind conditions present in most environments, it is usually difficult to exactly quantify sampling efficiency.
The main purpose of a well-designed sampler is to ensure that all or most of the particles in a given volume of ambient air are aspirated to the inlet and reliably transported onto a filter or through a dynamic sensor for analysis. The external geometry of the sampler may significantly affect aspiration efficiency. Particles may be lost during transmission through the sampler due to one or more physical mechanisms, such as direct wall impaction and gravitational settling, migration in the developing boundary layer, and electrostatic deposition. The main reason for such particle losses in the entrance region of a sampling inlet is the formation of a vena contract (for R&lt;1) and impaction of particles to the inner wall of the inlet. Thus, the concentration of particles collected on a filter or passed through a sensor is generally less than the aspirated particle concentration.
If there is a long distance between the entrance region and the sensor or collection surface, additional losses may occur, mainly due to gravitational settling and electrostatic deposition of particles. Gravitational settling depends on particle settling velocity and the distance from the inlet face to the filter or sensor surface. Electrostatic deposition depends on the electric charge on the particles and the electrical conductivity of the sampler's surface. The combination of these effects leads to non-uniform deposition of particles across a collection filter, which can lead to significant biases in the measurement of the deposited aerosol, e.g. from microscopic evaluation of selected portions of the filter.
In workplace environments, the protection of the workers' health is of primary importance and as such the size and concentration of the particles that can be inhaled by a person is of concern. International conventions have been agreed concerning the aspiration efficiency of different size particles in different portions of the human airway system, thus fostering the development of samplers that show similar aspiration efficiencies. Rather than attempting to sample "total dust", optimum sampling is defined on the basis of the efficiency of human breathing. The sampling efficiency for particles larger than 30 .mu.m was set at 50%. However, only limited information is available regarding the collection efficiency of particles larger than 20 .mu.m for currently used samplers.
2. Description of Related Art
No U.S. or foreign patent documents have been found with related claims. No patent document was found for the closed-face 25 mm filter cassette with which data were taken for comparison, shown in FIGS. 4a-4f.